Nitrogen levels in the Tully River--a long-term view.

Enhanced run-off of nutrients from agricultural development in catchments bordering the Great Barrier Reef (GBR) is regarded as a potential threat to inshore reefs. In an attempt to better understand the impact of river run-off, the Australian Institute of Marine Science (AIMS) initiated a sampling program to measure nutrient concentrations in rivers draining to the GBR shelf. Our longest data set is from the Tully River in the wet tropics, sampled in collaboration with the Bureau of Sugar Experiment Stations (BSES). After thirteen years, we ask whether there has been any change in river nutrient levels. Water samples were collected at monthly intervals, with additional, wet-season samples. In recent years, a trend of increase in nitrate (at low flow) and particulate nitrogen concentrations has been observed. In this period, a significant increase in agricultural activity has occurred within the Tully basin. The area under sugarcane and bananas has doubled and fertiliser nitrogen use has increased by 130%. Increased concentrations of nitrogen in the river system may be a consequence of this increase in agricultural activity.